Lithographic plates (also called lithoplates or printing plates) are sheet-like articles with metal, paper, or plastic substrates that have been cleaned, treated, and coated with a thin film of a light-sensitive material. This material is typically naturally soluble in some solvents, such as water, but becomes insoluble thereafter upon exposure to light. A photoimage is formed on the plate by placing a negative or positive image in close contact with the coated plate and then exposing the plate to a controlled light source such as ultraviolet light. The light passes through the positive or negative thereby polymerizing the coating and rendering the light exposed portion insoluble. The portions of the coating which remain shaded by the opaque portions of the negative or positive, and thus which are not exposed to light, remain soluble. Afterwards, the plate is usually developed in either an alcohol/water or an aqueous alkaline developing solution to remove the unexposed non-image portions from the plate
The film coating of light sensitive material mentioned above typically contains a polymeric backbone. The strength of the polymeric backbone is important because if it tends to be weak, the developing solution will degrade it and the quality of the developed image will be drastically lowered. Furthermore, the coating will be susceptible to solvent attack and further degradation during the on-line printing process.
In the past, such coatings have typically involved two separate layers, e.g. an oligomeric diazonium resin layer with an undercoat containing a urethane moiety. See, for example, U.S. Pat. No. 4,316,949. The undercoating was necessary in order to achieve adhesion of the image areas to the aluminum plate. Such bilayered coatings, though, present some drawbacks. First of all, if the urethane-based moieties are not polymerized enough, they will be susceptible to degradation by developing solutions, thereby hampering the quality of the developed photoimage. An additional drawback to such coatings is the high degree of tackiness they possess due to the use of viscous diols in the reaction process with diisocyanates to make urethanes. The viscous coating will quite often require the presence of a separate surface coating, such as polyvinyl alcohol, to detackify the urethane undercoating. If the undercoating is not detackified, it will be difficult to remove the negative from the presensitized plate. Furthermore, the presensitized plates would tend to stick to one another during the production process if the extra surface coating was not present.
Although specialized urethane-based coatings have been developed for applications other than in lithography, such as for use in the circuit and cable industries, these coatings would not necessarily be suitable for use in coating lithographic plates. For example, DE 3630954; DE 3630995; and DE 3630996 disclose protective coatings for circuit boards and the like which are the reaction products of olefin unsaturated monoisocyanates and novolak resins. Because novolak resins are generally non-heat reactive, there is not a great amount of cross-linking of the urethane reaction product. Consequently, these urethanes would not be suitable for use as coatings in lithographic articles since they would not serve as a strong backbone and thus would be susceptible to degradation by the developing solutions. Additionally, the polymeric material would be susceptible to further degradation by various solvents during the on-line printing process.
Many of the existing photosensitive precursors or oligomers, e.g. urethanes, are compatible primarily with only diazonium photosensitizers and alcohol/water developing solutions, both of which present drawbacks in their use. Diazo-based plates have a limited shelf life because the diazo compounds can degrade the aluminum plates and therefore require a silicated barrier coating between the diazo coating and the aluminum backing member. In addition, the diazo coatings present handling problems by reason of their sensitivity to heat, moisture, and tungsten light. The use of alcohol/water based developing solutions present both environmental and safety problems. Thus, there has been a preference in the industry for photopolymer precursors which are compatible with halomethyl-s-triazine-type photosensitizers and aqueous alkaline developing solutions.
In view of the foregoing, what is needed in the industry are photopolymer precursors which are non-tacky, easy to apply, and which have sufficient strength to resist degradation during both the development and on-line printing processes. Additionally, the photopolymer precursors should be compatible with the particular photosensitive compounds and developing solutions which are becoming increasingly used within the lithographic field.